How to fit a mortice lock

Monday, 2 July 2012 11:06 AM

Today’s Checkatrade expert is Ian Leah, owner 999 Locks.

A securely locked door is a homeowner’s first line of defence against unwanted intruders so it is essential to fit the correct type of locks and to keep them in good working order. An inappropriate or badly fitted door lock can compromise the security of a home, invalidate household insurance and in extreme cases might even put its occupants at risk.

Domestic security is one of those areas where it’s probably best not to compromise. For this reason it is usually best to contact your local locksmith, who will be able to advise you and replace your lock at a reasonable price. However, for DIY-savvy homeowners, here is a step-by-step guide on how to fit a British Standard five-lever mortice deadlock. While these instructions will suit the majority of locks, it is always best to check the manufacturer’s guide for proprietary instructions before starting work.

1. Choosing your lock
Many insurance companies now have minimum standards for door locks and will usually require a British Standard five-lever deadlock be fitted in a wooden front door. All British Standard locks are clearly marked on the lock and packaging with the BS Kite Mark.

Mortice deadlocks typically come in two depths and it’s important to get the correct one, especially if fitting to a glazed door or replacing an existing lock where it’s important to align the keyhole. In most cases the narrower lock (some manufacturers will quote a notional size of 2.5 inches) will be appropriate.

2. Cutting the mortice
A mortice lock is set into the edge of the door so it is necessary to cut a slot (mortice) in the edge of the door.

First, cut the deep slot for the lock body. Some locks will be supplied with a template to help you mark up the edge of the door. If not you will need to place the lock body against the edge of the door and mark up the dimensions of the slot. Cut the slot by drilling a series of vertical holes using a flat drill bit and then open up the slot with a small chisel, as seen in the picture.

The second stage is to cut the shallower mortice for the fore end of the lock to fit into. Either use the template supplied or place the lock body into the slot you’ve just cut, mark around the fore end with a sharp pencil, remove the lock and chisel out the shallower mortice.

3. Making the keyhole
If you don’t have a template you will need to mark the position of the keyhole before drilling the door. Position the lock in the mortice and mark the bottom edge of the case on the edge of the door. This is your reference point.

Drilling the keyhole Measure the distance from the front edge of the lock to the centre line of the keyhole (this is the back-set) and the height of the keyhole above the bottom edge of the lock case. Use the reference point to transfer these measurements onto the door and make a vertical slot on each side of the door with a drill.

Fit the lock into the edge of the door and make sure the keyholes you’ve just drilled line up with the lock. Then use the screws provided to fix the lock into the door.

Finally, fit the keyhole escutcheons with the small screws provided. If one of the escutcheons has a flap, fit this to the outside of the door as it helps to keep draughts and dirt out of the keyhole.

4. Fitting the keep
The final job is to fit the box keep into the frame.

Turn the key to throw the bolt and mark its position on the doorframe. Use these marks to determine the height of the keep in the frame.

Retract the bolt, fully close the door and note if the front edge of the door is flush with the frame. Measure from the front edge of the door to the centre line of the lock and use this measurement to determine how far back the keep should be from the front edge of the frame.

As before, make a slot in the frame (for the box part of the keep) by drilling a series of vertical holes and opening these out with a chisel. Position the keep into the slot, mark round it with a sharp pencil and use a chisel to cut out the shallow part of the mortice.

Before screwing the keep into place; position it in the frame, close the door and check that the lock operates properly.

Checkatrade provides this regular column for HouseFixer.co.uk. Checkatrade is a vetting service to find genuine trades people and combat rogue traders. For more information visit the Checkatrade website.